This invention relates to the art of manufacturing recording discs (phonorecord discs and video discs) and blanks from which such recording discs can be made utilizing known compression molding techniques. An aspect of this invention relates to layered recording discs comprising a core layer and at least one recording groove-accepting thermoplastic layer in adherent contact with the core layer. Still another aspect of this invention relates to the preparation of a layered sheet from which either recording disc-size blanks or relatively thicker, undersize blanks can be made by cutting the sheet into discrete segments. Still another aspect of this invention relates to the layered blanks themselves or layered recording discs produced from these blanks.
Most of the phonorecord discs and video discs produced to carry audio and/or video frequency recordings in their impressed or "stamped" grooves are presently manufactured from a vinyl polymer (including various vinyl copolymers and vinyl arene resins such as polystyrene). In the lexicon of the recording disc art, the term "vinyl" is most often used in reference to vinyl halide homopolymers and copolymers containing at least some vinyl ester units, e.g. polyvinyl chloride and its copolymers, particularly the vinyl chloride-vinyl acetate polymers having a major amount of vinyl chloride units. These polymeric materials are considered to have outstanding recording properties, including the ability to form recording (sound and/or video) grooves with excellent fidelity of sound or video reproduction. The vinyl arene polymers are typically referred to as the "polystyrenes" and, though attractive for economic reasons, are considered to be technically inferior to the vinyl chloride family of polymers, particularly from the standpoint of wear resistance. (The reason for the more favorable economics of the polystyrenes is their more ready adaptability to injection molding techniques.) Still another class of polymers utilized in recording disc manufacture is the acrylic family, particularly polymethylmethacrylate.
In the manufacture of recording discs from vinyl chloride-type polymers, the use of injection molding is not practical. The typical manufacturing techniques involve extrusion of a relatively formless, discrete mass or "biscuit" of hot polymer. The extruder is designed to make "biscuits" of essentially equal mass, whereby each "biscuit" contains just enough material to make a single recording disc (or a slight excess over this amount). The biscuit is conveyed to the pressing device, which applies heat and pressure to the biscuit while impressing the desired recording disc profile and recording grooves upon the hot polymer. Neither the biscuit-pressing technique nor the injection molding technique are well suited to making layered recording discs, although efforts have been made to make laminated phonograph records from a plurality of biscuits; see, for example, U.S. Pat. No. 2,039,140 (Barton), issued Apr. 28, 1936. Other examples of disclosures of laminated phonograph record structures drawn from the U.S. patent literature include: U.S. Pat. No. 3,072,519 (Salzman), issued Jan. 8, 1963, U.S. Pat. No. 3,687,769 (Dague), issued Aug. 29, 1972, and U.S. Pat. No. 3,768,946 (Matuschke), issued Oct. 30, 1973. Additional background information regarding phonograph record manufacture can be obtained from, for example, U.S. Pat. Nos. 2,529,083 (Keefe), issued Nov. 7, 1950, 3,663,136 (Westermann), issued May 16, 1972, and 4,034,033 (Carrere), issued July 5, 1977, and an article entitled "Phonograph Record Compositions" in Volume 15 of the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Interscience Publishers, New York, N.Y., 1967, pages 225 to 231; see also "Chemicals Set Records with `Oldies`", Chemical Week, Dec. 20, 1978, pages 51 and 52, and Journal of the Audio Engineering Society, vol. 25, pages 702 to 895 (1977), particularly pages 716 and 721, and 722.
As pointed out in the Chemical Week article, the vinyl chloride-type polymers account for 80% of the materials used in manufacturing records in the United States, though European firms and video disc manufacturers have shown considerable interest in acrylics. For the smaller, 45-rpm records, polystyrene is more commonly used in the United States, although the industry in some countries prefers vinyl chloride copolymers even for the 45's. A new formulation introduced recently comprises a styrene-vinyl aromatic-acrylic ester copolymer.
Lamination techniques have been used for decades in the manufacture of recording discs such as phonograph records. Prior to the advent of the vinyl resins or polymers in the manufacture of phonograph records, it was a fairly common practice to make phonorecords from a cheap core element and resinous layers on the top and bottom of this element. See, for example, pages 716, 721, and 722 of the previously cited Journal of the Audio Engineering Society. This type of layered record and its laminated modern variations (e.g. see the 2,039,140, 3,072,519, 3,687,769, and 3,768,946 patents cited previously) are generally considered more complicated to produce than either the injection-molded polystyrene record or the "vinyl" compression-molded record, both of which are made from discrete quantities of a single polymer or single polymer mixture. According to the aforementioned Chemical Week article, a British company has issued recording discs having decorative pictures included in a laminated structure. The picture is sandwiched between two pieces of film, then covered with polyvinyl chloride film. The process for making these records is said to have a tendency to present handling problems and to be slow and expensive.